The present invention relates to increasing the detection sensitivity for defects or other specific features in a structural material using ultrasonics.
Ultrasonics is used increasingly for nondestructively testing and inspecting parts of structural material for detecting defects, flaws, etc., in its interior. Ultrasonic echo sounding is actually a tool of long standing. Successful practicing of this method has raised hopes for broadening its use. Specifically, one does not merely wish to detect the relative timing of a return echo or the absorption of a penetrating ultrasonic wave or beam; rather it is deemed desirable to make full use of the fact that the interaction of an ultrasonic signal with the texture of the material in the general sense is quite complex, and the signals being received carry with them the complexity of that interaction.
One major aspect of interest is that the interaction of acoustic waves with the material is usually frequency selective, or to say it differently, properties of a structural material which affect in one way or another acoustic vibrations have dispersive characteristics or are frequency dependent otherwise. Of course, the resolution of a flaw detecting method using ultrasonic echo signals is inherently frequency selective due to diffraction. Diffraction, however, is a phenomenon which, at least in the first order, is quite independent of the propagative medium and of the type of waves used. The situation is quite different as regards dispersion and frequency dependency of absorption as these phenomenon are to a considerable extent dependent upon the type of material, its internal texture thermal state, etc., and each instance is uniquely related to the type of waves used, for example, elastic vibrations. In conjunction therewith, a significant problem arises from the frequency dependency of transduction that is to say neither the generation nor the immediate transmission, nor the reception of acoustic signals is constant with frequency. On the other hand, employment of broad banded signals is desirable for reasons of resolution, detection of dispersion, etc.